Housed SLRs
Here's my recommendation - you can read my reasoning below:
Any SLR camera from any of the major manufacturers, housed in whatever housing suits you and your budget, with a fisheye lens, a macro lens and a pair of flashguns.
My SLR kit. From left: Tokina 10-17mm fisheye zoom, Sigma 105mm macro, behind them is the flat port for the macro, Canon 350D body, Ikelite housing with dome port for the fisheye, Canon Battery charger, two Ikelite Ai strobes, flash synch cable, arms and 2 x AA size battery chargers (My Ai's have been converted to run on AA;'s not D cells, they're much smaller and lighter to carry and work very nicely thankyou). Total weight is around 13kg in air, but the rig is very slightly positively buoyant in the water.
Buying an SLR
Brands
Forget brand entirely. You'll very quickly find out that each camera maker has a vociferous following which sincerely believes that only Canon (Or Nikon, or Olympus, or ......) make cameras that are worth using, and that buying any other brand is a sign of photographic inability, and may even suggest criminal tendencies and a deviant nature on the part of their owners. In truth, an SLR image is an SLR image, and the lenses you use are more important than the make of camera body. The biggest lens ranges come from Canon and Nikon, but there are independant makers (Sigma, Tamron, Tokina and others) offering lenses in mounts to fit just about any SLR.
Image Stabilisation
The only thing you may like to consider before choosing a brand is the provision of image stabilisation in the camera body. Image stabilisation will allow you to hand-hold your camera at shutter speeds that are slower than would otherwise be possible. Most people, with some care, can hand-hold a camera when the shutter speed is the reciprocal of the focal length, such as a 28mm wide-angle lens at 1/30th sec, or a 200mm telephoto at 1/200th sec, and so on. With image stabilisation you'll be able to hand-hold at longer shutter speeds, say 1/8th sec or 1/50th sec respectively with the 28mm and 200mm lenses. This means you can use either a smaller aperture or a lower ISO for better image sharpness and quality, or take pictures in lower light levels than would otherwise be possible. IS, or whatever the individual camera makers call it, is a good thing. Olympus and Sony buiild IS into some of the cameras in their ranges, Canon and Nikon put IS in their lenses. Lenses with IS are more expensive than non-IS lenses, so you'll possibly pay more in the long run,
Sensor size
The only real decision you need to make is where in the range from your chosen maker to spend your money - entry-level, enthusiast or full-frame/professional? The higher up the range you go the better the build quality and the handling. On the other hand, two entry-level bodies will probably cost you les than one enthusiast body, and on the basis that anything you take under water will flood sooner or later a back up is good idea. Assuming sensor sizes are the same, image quality is going to be the same.
The latest trend in SLRs is to full-frame sensors. These are 24mm x 36mm, which is the same size as a frame of 35mm film. Most entry level and enthusiast models have smaller sensors, around 22.3mm x 14.9mm (Canon) or 23.6mm x 15.8mm (Nikon), or the micro-four-thirds sensors of 18mm x 13.5mm used by Olympus and some others. Image quality is better from the larger sensors, but not noticeably better unless you're making really big prints or shooting at very high ISOs. What you do gain is a bigger, brighter viewfinder, which isn't to be underestimated. Full-frame lenses can be used on smaller-sensor cameras, but crop-sensor lenses will not cover the frame of a full frame camera.
(I bought an entry level crop sensor Canon 350D. I'd love a full frame sensor camera with a bigger viewfinder, but not enough to pay four times the price of my current camera for it, and image quality from my camera is identical to the image quality of the twice-the price semi-pro model next up the Canon range as the sensor is the same)
A situation every photographer's buddy will be familiar with, waiting!
Housings
One thing you must be absolutely certain of is that the camera you like can be housed, and you might also like to know how much it's going to cost.
Housings come, very broadly, in two types - plastic and alloy. Plastic housings are cheaper - about £1000 with a port - are based on a standard box with the controls fitted to suit the different cameras to be housed, and are usually rated to 60m. Alloy housings are much more expensive - around £2-3000 with a port - but smaller, shaped more neatly to the camera inside, undeniably sexier (!!!) and some are rated as deep as 90 to 100m. Plastic housings are clear, so you can see if they're leaking, whilst alloy housings incorporate leak detectors.
(I bought an Ikelite plastic housing, and I'm very happy with it. If I changed my camera I'd buy the appropriate Ike housing without a second thought)
Ikelite housing body - most Ikelite housings will look alike as they're based around a plastic box with the controls added where they need to be placed to control a specific camera
Lenses and Ports
Macro lenses are designed to produce their best results when the subject is relatively close to the camera, and focus close enough for your subjects to appear life-size on the sensor. The most common macro lenses are 50mm and 105mm (Which are effectively 75mm and 160mm on crop sensor dSLRs). The shorter 50mm lens can mean getting very close for small subjects, making it tough to get your flashguns in place to light them, whilst the longer lens is a bit too long for medium sized fish and similar subjects - you'll be shooting through so much water the image will lose contrast and colour. Ideally, you need both, in practice, either will do, though the shorter lens might be easier to start with. Alternatively, Sigma make a 70mm macro that falls neatly between the two.
You need a flat port for macro. Make sure the port is long enough for you macro lens at full extension (As you focus on closer subjects the lens extends further from the camera body. This happens with all lenses, but the closer you get and the longer focal length lens you use the greater the extension will be.)
Flat ports for macro - you can easily see why they're called flat ports. Ikelite makes complete ports in different lengths for different lenses, other makers produce a port head and different lengths of extension tubes to adapt their ports for different lenses
(I have a Sigma 105mm macro with the appropriate Ikelite flat port. I'd sometimes prefer a 50mm lens, but on the whole I'm happy with what I've got)
Shot with the 105mm macro, this little blenny lives in a piece of dead coral on the fore-deck of Thistlegorm, his head is less than 5mm across
Wide-Angle lenses come in two flavours, fisheye and rectilinear, and the shorter the focal length the wider the angle of view. Here's the same scene shot with an 18mm lens (=28mm on full frame), a 10mm rectilinear wide-angle (=16mm on full frame) and a 10mm fisheye (=16mm fisheye on full frame).
Rectilinear lenses are designed to produce an image where straight lines stay straight, and maximum angle of view will be around 110 degrees. Full frame users will need 20mm or shorter, crop sensor users will need one of the 10-20 (ish!) zooms, either makers own or independant.
Fisheye lenses are designed to give the maximum possible field of view by allowing the image to distort, producing pictures where straight lines that don't pass through the centre of the frame are bowed outwards. Go for a full-frame fisheye with a field of view of 180 degrees corner to corner and not a 'true' fisheye that produces a circular image. Full-frame users will need a full-frame fisheye of aroud 15mm, either from the camera maker or an independant, crop sensor users will need either the makers full-frame fisheye (Nikon or Olympus) or an independant (Any brand except Olympus, who use a smaller sensor than other makers). The new Tokina 10-17mm fisheye zoom is a remarkable lens and offers Canon users a true full frame fish for the first time, though it can be had to fit Nikon and others as well.
Straight lines are rare underwater, so the extra angle of view of a fisheye, which means you can get closer and have less water between you and the subject, has no real downsides. Some people feel that wrecks, which do have straight lines, are better shot using a rectilinear wide-angle, I use a fisheye.
Afterdeck of Salem Express, the picture gets its impact from the lines, and careful choice of viewpoint has minimised, though not completely eliminated, the curved-line effect of the fisheye
Wide-angles require dome ports. Water is denser than air, so light is diffracted as it passes from water to air. The result of this is to make things underwater appear larger and closer than they actually are, something so familiar to divers they don't notice it. The problem for lenses is that they do notice it, and the effect is to increase focal length by about a third, so your expensive wide-angle suddenly isn't very wide any more. Lenses wider than 35mm (35mm equivalent, on a crop sensor camera the actual focal length will be 24mm or less) suffer from distortion and colour fringing problems if used behind a flat port.
Three different dome ports from Ikelite - the curved bit at the front is the same on each, but the length of the stalk attaching them to the camera body is different. The shortest port is intended for very compact and very wide-angle lenses and is too short to allow a lens shade to be fitted. As with macro ports, other makers offer the port head and extension tubes to fit different lenses
Dome ports work underwater by producing an image of the subject on which the camera lens focuses. An image of a subject at infinity will be formed at three times the radius of curvature of the dome from the outside of the dome. Dome radius is usually between 3" and 4", meaning the image formed by the dome will be between 9" and 12" outside the dome. There will also be something like 6" between the inside of the dome and the sensor plane of your SLR, so the lens needs to be capable of focusing down to about 15" or 18" to focus on distant subjects. Subjects underwater are going to be much closer than infinity, so your lens must be able to focus closer still, the near limit is the dome itself, at about 6". In practice, your wide-angle must focus to 12" / 300mm, if it won't you can add a close-up lens. Either a +3 or +4 dioptre will do it (The close-up lens will reduce the maximum distance on which the camera lens will focus to a distance which is equivalent to 1 metre divided by the power of the close-up lens, so adding a +3 allows the camera to focus on 33cm and +4 will focus on 25cm.)
Your housing maker will provide a range of both flat and dome ports, and will have a list of lenses and suitable ports on their web-site.
(I use a Tokina 10-17mm Fisheye Zoom, with which I'm very happy - it gives me a full-frame fisheye effect when set to 10mm and a very wideangle without too much distortion when set to 17mm, though it's never rectilinear)
Flashguns (Strobes)
Back in the olden days all flashguns were controlled according to the Nikonos protocol and gave automatic exposure measured through the lens, TTL flash. When the flashgun went off the amount of light reaching the film-plane was measured and the flashgun extinguished when enough had arrived to give an accurate exposure. Digital sensors don't allow that simple and universal system to work (Or so the camera makers tell us, Fuji got it to work in their now discontinued S2 SLR), so you have a choice, either use the flashguns provided by your housing maker, who will include conversion circuitry to allow the electronics of you camera to talk to their own brand flashguns, or use manual flash. Going manual opens up a world of elderly but otherwise entirely acceptable flashguns, all available for a song on eBay 'cos everyone wants the new digital compatible TTL versions and are willing to pay megabucks for them.
Flash coverage is a thorny issue, with all the makers claiming their guns provide very wide and very even light coverage. In practice all guns give less illumination at the edge of their beam, and very few get close to providing 100 degrees of coverage, so you'll need two strobes to cover either a superwide rectilinear or full-frame fisheye. Having two guns also lets you be more creative with your lighting. After all, you can always turn one strobe off, but if you don't have a second gun you can't turn it on.
Macro can be easier to light than wide-angle, but flat and dull lighting is very common, see the Compact Macro page for more on lighting.
Regard whatever cabling that goes between your housing and flashguns as expendable and liable to fail at any minute. Carry a spare.
(I use a pair of rather elderly non-digital Ikelite strobes that came from eBay, they're not very adjustable - just full and half power - but the light from them is soft, even and with good coverage.)
Here's my full outfit ready to shoot wide-angle with a fisheye lens, appropriate port and twin strobes
Strobe positioning is a big subject, see the shooting techniques page!